Audio faders are well-known in the prior art and are used to adjust the gain of one or more audio signals in the production of a recording of a sequence of audio events. A common prior art apparatus for controlling the gain of the audio signal includes a movable control lever, slider or knob (collectively referred to hereinafter as a "lever") which is connected to a linear potentiometer. Movement of the lever, usually in a one-dimensional, linear movement, changes the electrical characteristics of the potentiometer. The potentiometer is electrically connected by well known techniques across a voltage supply having a magnitude representing the range over which it is desired to vary the control signal applied to the control input of an associated voltage controlled amplifier (VCA), also well known in the prior art.
Generally, in this type of prior art device, the potentiometer has its wiper arm connected directly to a fader control lever in order to vary a parameter such as the gain of an audio signal. In use, the operator manually adjusts the position of the lever which is coupled to the wiper arm of the potentiometer. This action adjusts the control voltage applied to the gain control input of the voltage controlled amplifier. Such a prior art analog potentiometer system is described in U.S. Pat. No. 4,429,219, issued Jan. 31, 1984 to Yochum et al. A problem associated with prior art mechanically-based potentiometers, according to the description given in the Yochum et al. patent (col. 3, lines 15-23), is that rubbing of the lever's wiper arm within the linear potentiometer causes the mechanical assembly to wear, and this in turn introduces noise into the gain control signal provided at the wiper arm. The noise produced by the potentiometer is combined with the audio signal entering the voltage-controlled amplifier. Consequently, the gain control operation introduces an undesired noise component into the audio signal, causing audio signal inaccuracies and distortions.
Today, a transition from analog-based audio processing consoles to digital-based consoles for audio mixing and recording applications is occurring. Accordingly, in order to use the prior art analog potentiometer type of fader control discussed above, analog to digital converters are necessary to convert the analog position signal from the potentiometer and its related lever to a digital signal, thus adding more circuitry and adding a potential for further errors and noise in the system.
Therefore, it is desirable to utilize in digital-based audio consoles a digitally-based audio fader system. The Yochum et al. patent described above, as well as U.S. Pat. No. 4,412,182 issued Oct. 25, 1983 to Yochum, disclose audio fader systems which do not rely on mechanical contact between the elements for establishing the position detection operation. Instead, a shutter and light beam arrangement is employed which is described in the Yochum et al. patent (col. 4, lines 50-57) as being essentially immune to the wear problems associated with the prior art analog potentiometer system. Furthermore, the bulk of the signal processing may be accomplished with digital circuitry permitting, according to the Yochum et al. patent, low noise operation. In the Yochum et al. patent, a number of light emitting diodes or light sources are interfaced with a shutter-like device or other obturator and a photo-sensitive detector. As disclosed in the Yochum et al. patent, there are 13 emitter/detector pairs and thus 13 signals whose values collectively identify the location of the shutter over the range of 66 possible positions (col. 7, lines 6-18). The shutter is connected directly to a manually movable lever.
The prior art also includes a rotatable fader knob attached directly to the shaft of an optical encoder, such that rotation of the knob rotates the optical encoder shaft, which, with appropriate circuitry, controls the gain, for example, of an audio signal.
Other light-based position or channel detection and selection apparatus is shown in the following U.S. Pat. No.: 4,015,253 issued Mar. 29, 1977 to Goldstein; U.S. Pat. No. 4,137,451 issued Jan. 30, 1979 to Einolf; U.S. Pat. No. 4,122,395 issued Oct. 24, 1978 to Schotz et al.; U.S. Pat. No. 3,835,384 issued Sep. 10, 1974 to Liff; U.S. Pat. No. 3,755,681 issued Aug. 28, 1973 to Montross; U.S. Pat. No. 3,381,288 issued Apr. 30, 1968 to Van Vlodrop; U.S. Pat. No. 3,619,626 issued Nov. 9, 1971 to Rudolph; U.S. Pat. No. 4,573,110 issued Feb. 25, 1986 to Drumm; and U.S. Pat. No. 3,363,106 issued Jan. 9, 1968 to Park.
The apparatus described in the Yochum et al. patent, while perhaps an improvement over the prior art analog contact-type potentiometer system, has certain drawbacks and disadvantages. For example, the system requires a number of light-emitting elements and light-detecting elements, specifically 13 emitter/detector pairs, which will produce 13 digital signals whose values must be interfaced in order to provide a signal which corresponds to a value of the movable lever in any one of a number of positions (col. 7, lines 6-18).
In addition, the Yochum et al. patent does not disclose any means by which the lever may be automatically moved to a selected position, although this type of mechanism is well known in the audio fader prior art. In these devices, in further editing of an audio sequence, the fader lever or levers are caused to move to the position(s) the operator had adjusted them to during previous mixing or editing passes in order that the operator may visually observe the previous positioning(s) of the fader levers. This acts as a visual indication to the operator and assists him/her in adjusting and further editing the audio sequence. Thus, while the Yochum et al. patent broadly discloses an apparatus which may be seen as a logical extension from an analog audio fader to a digital audio fader, the apparatus described in the Yochum et al. patent has the disadvantages set forth above. What is desirable, therefore, is a system which is digitally-based, but has few parts, provides good tactile feedback to an operator, is accurate and allows for automatic movement of the fader lever knob.